An example of effective methods for reducing the power consumption of an electronic device is to reduce the operation voltage of the device. However, if the operation voltage of a conventional MOS transistor is excessively reduced, a required current value cannot be availed, so that the operation speed of the transistor is significantly decreased.
Therefore, there has been considered a tunnel transistor in which a tunnel current flows through a PN junction between a source region and a channel region. The current value of the tunnel transistor is determined by the PN junction through which the tunnel current flows. Among the all portions of a gate electrode of the tunnel transistor, a portion in the vicinity of the PN junction contributes to the control of the tunnel current. On the other hand, a gate insulator of the tunnel transistor under the other portions of the gate electrode serves as a parasitic capacitor of the tunnel transistor. The parasitic capacitor causes a decrease In operation speed of the transistor. Therefore, it is desired to reduce the parasitic capacitance of the tunnel transistor.